Teleology is the study of ends and goals, things whose existence or occurrence is purposive. Aristotle’s views on teleology are of seminal importance, particularly his views regarding biological functions or purposes. This article surveys core examples of Aristotle’s invocations of teleology; explores philosophically puzzling aspects of teleology (including their normativity and the fact that ends can, apparently, act as causes despite never coming to exist); articulates two of Aristotle’s arguments defending commitment to teleology against critics who attempt to explain nature solely through appeal to nonteleological efficient and material causes; and argues that Aristotle was an ontological realist when it came to teleology: he conceived of ends as an irreducible and ineliminable aspect of the causal structure of reality. Other interpretive controversies are addressed more briefly.